InputGestureManager: Add shortcut for keyboard backlight adjustment

This commit adds the shortcut of meta+brightness_up/brightness_down to
increment and decrement the keyboard backlight brightness accordingly.

Bug: 401626732
Test: Build and flash DUT and verify that pressing meta+brightness
up and down keys adjust the keyboard backlight brightness.
Flag: com.android.hardware.input.keyboard_backlight_shortcuts

Change-Id: I5aed502788cb34d7f554665caedc201c5a225834